The tumor microenvironment is considered to be an important factor in tumorigenesis and tumor progression and metastasis. In view of the fact that 70% of patients with stage 4 and stage 4s neuroblastoma have metastatic disease in the bone marrow and 56% in the bone at the time of diagnosis, our project has been focused on the study of the bone marrow microenvironment in neuroblastoma progression. Over the last 5 years, we have identified a pathway at the center of the interactions between neuroblastoma cells and bone marrow-derived mesenchymal cells (BMDMCs). We observed that neuroblastoma cells in the bone marrow microenvironment produce Galectin-3 binding protein (Gal-3BP) that interacts with Galectin 3 present in BMDMCs and stimulates the production of Interleukin-6 (IL-6) by these cells. IL-6 has multiple effects in the bone marrow. It stimulates osteoclasts, enhances neuroblastoma cell proliferation and protects them from drug-induced apoptosis. Furthermore, in collaboration with project 2, we have shown that th Gal-3BP/Gal- 3/IL-6 pathway also controls the interaction between neuroblastoma cells and monocytes that infiltrate primary tumors. On the basis of these data, we hypothesize that the bone marrow provides tumor cells with a sanctuary against the cytotoxic effects of chemotherapy via a Gal-3 BP/Gal-3/IL-6 pathway. Consequently, we hypothesize that targeting this pathway would be of therapeutic benefit in patients with neuroblastoma and bone marrow and bone metastasis. This project has 3 specific aims: Aim 1, will examine the mechanism by which Gal-3BP stimulates the transcriptional expression of IL-6 in BMDMCs. Aim 2 will examine the mechanism by which IL-6 protects tumor cells from drug toxicity with a particular focus on the role of anti-apoptotic proteins like survivin, XIAP, Bcl-2 and Bcl-{XL} that are upregulated by IL-6 in neuroblastoma cells and on signaling pathways (STAT-3, ERK1/2 and Akt) downstream of IL-6 signaling. In aim 3, in collaboration with project 2, 3 and 4, we will examine the effect of genetic ablation of IL-6 on neuroblastoma initiation and progression and the effect of targeting the Gal-3BP/Gal-3/IL-6 pathway on neuroblastoma progression in preclinical models. The focus of this project is therefore on the understanding of the molecular mechanisms that control the interaction between neuroblastoma cells and the bone marrow microenvironment and on using this information to identify large and small molecules that can be tested in preclinical trials by our project and in clinical trials by project 4 and the NANT.